The present invention relates to an inverter circuit for use in a miniaturized cold-cathode fluorescent lamp, etc., a miniaturized transformer suitable for such an inverter circuit and a discharge tube glow circuit including such an inverter circuit.
Liquid crystal displays used in notebook-type or laptop-type personal computers, electronic notebooks, portable telephone sets, etc. comprise cold-cathode fluorescent lamps (CCFL), light-conducting plates (edge light-type light source panels), etc. for backlight. To energize these light sources, an inverter circuit is necessary as a power supply.
The CCFL is suitable as a backlight for a liquid crystal display because it has a high efficiency and easily emits white light. However, since the CCFL needs an inverter circuit generating high voltage, it has not easily been assembled into a miniaturized portable electronic equipment because the miniaturization of the inverter circuit is limited. As a result, photodiodes were used in various kinds of miniaturized portable electronic equipment in many cases.
A conventional inverter circuit for the CCFL comprises a booster transformer including a ferrite core, and the booster transformer occupies a considerable space in the inverter circuit, preventing the inverter circuit from being miniaturized. Examples of such inverter circuits and transformers are shown in FIGS. 4(a) and 4(b). As shown in FIG. 4(a), the secondary winding side of the transformer 1 is connected to a discharge tube 3. The transformer 1 comprises a closed magnetic circuit-type ferrite core provided with a primary winding and a secondary winding. Accordingly, the miniaturization of the transformer 1 has been difficult, leaving the inverter circuit large in size. Also, since the discharge tube 3 has negative resistance characteristics opposite to those of general resistors in the relation of voltage and current, guide current is stabilized by a ballast capacitor 2 disposed between the discharge tube 3 and the transformer 1, and the ballast capacitor 2 serves to prevent the miniaturization of the inverter circuit.
With respect the above problems, it was reported that the inverter circuit can be miniaturized by using as a booster transformer a magnetic flux leak-type transformer having weakly coupled primary winding and secondary winding. Examples of such inverter circuits and magnetic flux leak-type transformers are shown in FIGS. 5(a) and 5(b). In a magnetic flux leak-type transformer 6 whose coupling coefficient drastically changes depending on a load, as the current increases after the start of discharge, voltage decreases and current increases to a stable stationary state at the secondary winding, thereby making the ballast capacitor 2 unnecessary. Since the ballast capacitor is the most likely cause to troubles, the omission of the ballast capacitor contributes to the improvement of reliability of the inverter circuit.
However, under circumstances that further miniaturization and reduction in thickness are desired in liquid crystal displays assembled in note-type or laptop-type personal computers, electronic notebooks, portable telephone sets, etc., the CCFL used for backlight appears to fail to provide enough miniaturization as long as a transformer of the CCFL is composed of ferrite.
Specifically, when magnetic cores of ferrite are made thin or small, they show deteriorated soft magnetic properties, failing to obtain the desired characteristics, and are likely to be broken. Also, since ferrite has a low saturation magnetic flux density and a relatively low effective permeability, magnetic cores made of ferrite cannot be made small drastically. Further, since the magnetic cores of ferrite have poor temperature characteristics, the performance of the transformer changes from an initial stage in which the temperature of the magnetic core is low to a stage after a certain period of time in which the temperature of the magnetic core is elevated.